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The present invention relates to optical sensors, and in particular to a high-resolution chemical sensor using a waveguide-coupled microcavity optical resonator.
During the past few years, a substantial amount of research has been performed in the field of optical microcavity physics, in order to develop high cavity-Q optical microcavity resonators. In general, resonant cavities that can store and recirculate electromagnetic energy at optical frequencies have many useful applications, including high-precision spectroscopy, signal processing, sensing, and filtering. Many difficulties present themselves when conventional planar technology, i.e. etching, is used in order to fabricate high quality optical resonators, because the surfaces must show deviations of less than about a few nanometers. Optical microcavity resonators, on the other hand, can have quality factors that are several orders of magnitude better than typical surface etched resonators, because these microcavities can be shaped by natural surface tension forces during a liquid state fabrication. The result is a clean, smooth silica surface with low optical loss and negligible scattering. These microcavities are inexpensive, simple to fabricate, and are compatible with integrated optics.
Optical microcavity resonators have quality factors (Qs) that are higher by several orders of magnitude, as compared to other electromagnetic devices. Measured Qs as large at 1010 have been reported, whereas commercially available devices typically have Qs ranging from about 105 to about 107. The high-Q resonances encountered in these microcavities are due to optical whispering-gallery-modes (WGM) that are supported within the microcavities.
As a result of their small size and high cavity Q, interest has recently grown in potential applications of microcavities to fields such as electro-optics, microlaser development, measurement science, and spectroscopy. By making use of these high Q values, microspheric cavities have the potential to provide unprecedented performance in numerous applications. For example, these microspheric cavities may be useful in applications that call for ultra-narrow linewidths, long energy decay times, large energy densities, and fine sensing of environmental changes, to cite just a few examples.
In particular, a significant potential application for microcavity resonator devices is chemical/biological agent sensing. Chemical sensors known in the art include MEMS (microelectromechanical systems) chemical sensors, optical waveguide-based sensors, surface plasmon resonance (SPR) chemical sensors, surface acoustic wave (SAW) chemical sensors, mass spectrometers, and IR (infrared) absorption spectrometers. The sensitivities of many types of chemical sensors are limited by the small interaction region. On the other hand, spectrometers characterized by a high sensitivity, such as spectrometers using mass or optical detection properties, are large devices, and do not provide the advantages of miniaturized sensors, such as prior art MEMS sensors. The compact size of miniaturized sensors provide significant advantages. For example, miniaturized sensors are well adapted for in situ functioning. Also, miniaturized sensors would be small enough to be deployed in large numbers and implemented for remote probing.
It is therefore desirable to provide chemical sensors with an improved detection sensibility and resolution, while maintaining the compact size of miniaturized sensors, such as the MEMS sensors known in the art.
The present invention is directed to a miniaturized chemical sensor featuring an optical microcavity coated with a surface layer, and a waveguide that evanescently couples light into the microcavity. The surface layer interacts with at least one molecule species. The interaction alters the evanescent light coupling properties, typically through a change in refractive index of the microcavity. The waveguide is preferably a SPARROW (Stripline Pedestal Anti-Resonant Reflective Optical Waveguide) waveguide chip.
A chemical sensor constructed in accordance with the present invention includes a substrate, an optical waveguide disposed on the substrate, and an optical microcavity. The optical waveguide evanescently couples light into the microcavity. In one embodiment, the optical waveguide is an interferometric waveguide, including three waveguide arms: 1) an input channel for input coupling light into the microcavity; 2) a drop channel for out-coupling light from the microsphere into the waveguide; and 3) a reference channel that does not interact with the microcavity.
The optical microcavity is coated with a surface layer adapted to chemically interact with at least one molecule species. In one embodiment, the molecule species may be a molecule in a fluid or a gas. For example, the molecule species may be a molecule found in a chemical vapor surrounding the microcavity, by way of example. Alternatively, the molecule species may be a molecule found in a liquid solution. The chemical interaction causes a change in the index of refraction of the microcavity, and thus in the resonant frequency of the microcavity. By measuring the resonant frequency shift, or by measuring the phase difference readout caused by the change in the index of refraction, the molecule species may be detected.
A miniaturized chemical sensor constructed in accordance with the present invention provides a significantly increased sensitivity, as compared to prior art sensors. In one embodiment of the invention, a refractive index sensitivity of about 10xe2x88x9211 is achieved, representing an improvement of over three orders of magnitude, as compared to prior art waveguide chemical sensors.